1. Field of the Invention
The present invention relates to a developer supply technique for use in a developing device which develops an electrostatic image by a developer in an electrophotographic copying machine or an electrophotographic laser beam printer. Furthermore, the present invention relates to an image forming apparatus, such as an optical printer and for example, a copying machine or an electrophotographic laser beam printer, which adopts a developing device utilizing such a developer supply technique.
2. Related Background Art
Among conventional developer supply devices of this type, a developer supply device is generally known which detects a toner amount (a mixture ratio of a carrier and a toner in the case of using a two-component developer) in a developing device by sensing permeability, reflectance or change in the amount of the toner and controls a toner supply amount from the side of a toner hopper so as to maintain a constant toner amount. For example, in case the reflectance is measured with a two-component developer, a method, which performs the following control operation, is adopted in order to maintain the amount of the toner in the developer: since the reflectance will vary in accordance with the mixture ratio of a carrier and a toner, a photodiode receives the light reflected from the developer and the output voltage of the photodiode is compared with a predetermined standard voltage. If the output voltage is lower than the standard voltage, a toner is supplied from a toner hopper for a predetermined period or in a predetermined amount, and if the output voltage is equal to or higher than the standard voltage, the toner supply is stopped.
However, as for the toner amount control of a toner supply device in an image forming apparatus, such as a copying machine, if the state quantity, such as room temperature, humidity, the number of copy sheets, document density and so on is changed, it is impossible to obtain a sufficient image only by supplying a toner of a constant amount or of an amount so as to increase the output voltage to the standard voltage.
If the density of an original is high and the number of copy sheets is large, the amount of toner consumed in the developer is large. Accordingly, the toner supply from a toner hopper cannot keep up with the amount of toner consumed and this sometimes causes a serious image defect, such as lowering of the image density, and, in an extreme case, a white blank or void wherein a part of the image does not exist. On the other hand, if the amount of the toner supplied from the toner hopper for a predetermined period is increased in order to adapt to the above-described condition, or reversely, if the control is made to supply the toner when the smaller amount of toner is consumed, too much toner is contained in the developer and this causes a deterioration of the image, such as a change in the image density, or, in an extreme case, blooming.
Furthermore, if the humidity and/or temperature inside the image forming apparatus is changed, the flowability or charging characteristics of the toner are also changed. Therefore, even if a constant amount of toner is supplied in response to the change in the toner density, the quality of the developed image is sometimes changed. In addition, if deterioration of the toner or the carrier, that is, the developer, and so on is caused by long-term use of the image forming apparatus, a similar problem also sometimes arises.